The present invention concerns a means for saving energy in a hydraulic circuit in which pressurized oil supplied by a hydraulic pump is used to activate various functions with the aid of one or several valves controlling these functions. Such a hydraulic circuit is especially suited for use in agricultural tractors. A tractor usually incorporates a system in which the hydraulic control circuit of the differential lock, the power take-off shaft, the high-speed gear or another equivalent hydraulically operated clutch has been connected to the lubricating circuit of the tractor's mechanical shifting gear. The pressurized oil to such a circuit comes from a hydraulic pump driven by the engine.
Since in such an arrangement the coupling elements require a pressure on the order of 1.5 MPa, and only a liquid flow sufficient to fill the operating cylinder, while for the lubrication of the shift gears is required a sizeable flow at a pressure not higher than 0.4 MPa, there must be some kind of throttling point in the system before the gear lubricating circuit, to raise the pressure to be proper for the said coupling means. A 1.5 MPa pressure regulating valve, for instance, is proper to be used to do this. However, this entails that the lubricating oil for the shift gears has to be driven against 1.6 MPa back pressure all the time, and this involves unnecessary waste of energy as well as need to cool the oil.
To overcome this drawback, a pressure-controlled free circulation valve by-passing the pressure regulating valve has been used, said valve being switched into action with the aid of the pressure rising in the circuit in question when none of the coupling valves is in operation. A solution of this type however requires a pressure accumulator operating in the range of 1.6 to 2.0 MPa. This accumulator is an expensive piece of equipment and susceptible to damage.
It is also conventional to employ three-way directional valves which have been dimensioned liberally enough to enable oil to be driven through them continuously. No separate free circulation valve is then needed because the directional valve can be set in the free circulated position. This design is however bulky and costly.